This Zen Cart module adds a nice lightbox effect when a shopper clicks on the product images or the "larger image" link on your product pages, as well as other pages. Upon clicking the available larger images are displayed in a very professional lightbox format with a link to close it. While the image lightbox is open the rest of the page is hidden behind the larger image in a black smokey, partially transparent whole screen effect to highlight the image.
Zen Lightbox can be used and is compatible with ImageHandler2 (most versions) and can display even additional product images in the lightbox. So you can have the great caching from image handler as well as the hover pop ups on small images and the light box effect. In fact you can even call to the lightbox script by hand by adding the "rel" attribute to any image link within your Zen Cart.